Many different types and styles of memory exist to store data for computers and similar type systems. For example, random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), read only memory (ROM), programmable read only memory (PROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash memory are all presently available to accommodate data storage.
Each type of memory has its own particular advantages and disadvantages. For example, DRAM and SRAM allow individual-bits of data to be erased one at a time, but such memory loses its data when power is removed. EEPROM can alternatively be easily erased without extra exterior equipment, but has reduced data storage density, lower speed, and higher cost. EPROM, in contrast, is less expensive and has greater density but lacks ease of erasability.
Flash memory, has become a popular type of memory because it combines the advantages of the high density and low cost of EPROM with the electrical erasability of EEPROM. Flash memory can be rewritten and can hold its contents without power, and thus is nonvolatile. It is used in many portable electronic products, such as cell phones, portable computers, voice recorders, etc. as well as in many larger electronic systems, such as cars, planes, industrial control systems, etc.
Flash memory is generally constructed of many memory cells where, generally, single bits of data are stored in and read from respective memory cells. The cells are generally programmed by hot electron injection and erased by Fowler-Nordheim tunneling or other mechanisms. As with many aspects of the semiconductor industry, there is a continuing desire and effort to achieve higher device packing densities and increase the number of memory cells on a semiconductor wafer. Similarly, increased device speed and performance are also desired to allow more data to be stored on smaller memory devices.
Individual flash memory cells are organized into individually addressable units or groups, which are accessed for read, program, or erase operations through address decoding circuitry. The individual memory cells are typically comprised of a semiconductor structure adapted for storing a bit of data and includes appropriate decoding and group selection circuitry, as well as circuitry to provide voltages to the cells being operated upon.
The erase, program, and read operations are commonly performed by application of appropriate voltages to certain terminals of the memory cell. In an erase or write operation the voltages are applied so as to cause a charge to be removed or stored in the memory cell. In a read operation, appropriate voltages are applied so as to cause a current to flow in the cell, wherein the amount of such current is indicative of the value of the data stored in the cell. The memory device includes appropriate circuitry to sense the resulting cell current in order to determine the data stored therein, which is then provided to data bus terminals of the device for access by other devices in a system in which the memory device is employed.
Programming circuitry controls a bit of a cell by applying a signal to a wordline, which acts as a control gate, and changing bitline connections such that the bit is stored by the source and drain connections. Erasing is performed as a blanket operation wherein an array or sector of cells can be simultaneously erased and typically produces a lower threshold voltage in the cell.
After a number of program and erase cycles of a flash memory, a concentration of electrons tend to accumulate in the charge trapping structure, which interferes with the ability to achieve a low threshold state, and limits the endurance and reliability of the device. Band-to-band tunneling induced hot hole injection can be used to erase cells. However, erasing by the hot hole injection may cause oxide damage, leading to charge loss in the higher threshold cells and charge gain in the lower threshold cells. Moreover, the erase time must be increased gradually during program and erase cycling (cycling) due to the hard to erase accumulations of electrons in the charge trapping structure. This accumulation of charge may occur because the hole injection point and electron injection point do not coincide with each other, and some electrons remain after the erase pulse. Thus, the total amount of time required to erase a sector of memory also increases with program and erase cycling.
In addition, during the sector erase of a flash memory device, the erase speed for each cell is different because of process variations (such as channel length variation). This difference in erase speeds results in a large Vt distribution of the erase state and the threshold voltage level of the erased sector. Because of this increasing Vt distribution of the erase state, some cells become harder to erase while some become over-erased. Thus, over continued cycling, a target threshold voltage Vt becomes more difficult to achieve, and poor endurance or reliability is observed.
Regardless of the flash architecture employed, reliably and accurately programming or erasing flash memory cells, and in particular dual sided ONO flash and multi-level flash cells can be particularly sensitive with the attendant complications of maintaining narrow Vt distributions in order to accurately read and determine a data state from a corresponding Vt level. In addition, even if such narrow distributions are attained for the various multiple levels, unless sectors of memory cells can be programmed and erased to within the acceptable limits quickly, efficiently, and reliably, little competitive advantage may be gained.
In view of the foregoing, a need exists for an improved method of quickly and efficiently erasing a sector or array of the various types of flash memory cells, in a manner that maintains a high reliability and narrow Vt distribution of the erased bits over extended program and erase cycling of the device.